An Algol star is a type of eclipsing binary, named after the prototype, Algol, that has periods of constant or near-constant brightness between minima, indicating that the two stars form a close binary of the detached or semidetached kind. Thousands of examples are known, with periods ranging from about five hours to 30 years and brightness variations of up to several magnitudes. In most cases where mass transfer takes place, it is by direct accretion rather than by an accretion disk. However, an extreme group of eclipsing, mass-transferring binaries, known as W Serpentis stars or "hyperactive Algols", do have accretion disks and may be in a pre-Algol-type stage. Algol stars are among the most important kind of star systems in terms of the information they provide on stellar masses and sizes.